Production of high-density, dead-burned magnesia in a shaft kiln

ABSTRACT

Dead-burned magnesia density is improved by controlling the heating in a shaft kiln to a rate of less than about 30* C. per minute through a temperature range of from about 1,100* to 1,700* C.

United States Patent Mayer et al.

[ 1 Feb. 15, 1972 [541 PRODUCTION OF HIGH-DENSITY,

DEAD-BURNED MAGNESIA IN A SHAFT KILN [72] inventors: Raymond P. Mayer; Ivan M. Thompson; Walter P. Zbojniewicz, all of Ludington,

[21] Appl. No.: 15,901

[52] US. Cl ..263/52 ....C04b 35/04 Field of Search ..263/29, 52 53 [56] References Cited UNITED STATES PATENTS 3,280,228 10/1966 Pack et a1. ..263/53 3,221,082 11/1965 beatham et al. ..263/53 Primary Examiner-John J. Camby Attorney-Griswold and Burdick, Stephen S. Grace and William R. Norris ABSTRACT Dead-bumed magnesia density is improved by controlling the heating in a shaft kiln to a rate of less than'about 30 C. per minute through a temperature range of from about 1,l00 to 1 ,700 C.

3 Claims, No Drawings PRODUCTION OF HIGH-DENSITY, DEAD-BURNED MAGNESIA IN A SHAFT KILN BACKGROUND OF THE INVENTION The present invention relates to the production of deadburned magnesia in a shaft kiln. Conventionally, magnesia (MgO), obtained for example by calcining magnesium hydroxide or. magnesium carbonate, is compacted into briquettes and fed into the top of-a gas-fired shaft kiln. The briquettes pass through apreheat zone, e.g., room temperature to l,l C.; a hot zone, e.g., 1,100 C. to dead-burning temperature (in excess of 1 ,700 C.); and a cooling zone. The dead-burned briquettes are continuously removed from the bottom ofthe kiln.

One particular use of the dead-burned magnesia is in. making basic refractory products, e.g., periclase brick. A highly desirable property of the dead-burned magnesia for this application is high bulk density. This will in turn produce a high density refractory product.

An object of the present invention is to provide a method of improving the density of dead-burnedmagnesiaprepared in a shaft kiln.

THE INVENTION Applicants have discovered thatthe density of the deadburned magnesia can be improved by heating the magnesia at a rate of less than about 30 C. per minute through a temperature range of from about l,l00 C. to l,700 C. Heretofore, in shaft kiln operation, the heating rate through this hot zone has been substantially higher, e.g., in excess of 100 C. per minute. However, if heating is carried out in accordance with the present invention, substantial improvement in product bulk density will be achieved.

The prescribed heating rate can of course be obtained by several means, e.g., control of (l) the briquette feed rate through the critical temperature range, (2) the external heating means (usually gas-fired jets), or (3) the rate and temperature of gases passing up through the shaft kiln.

In the practice of the present invention, the magnesia briquettes are fed into the shaft kiln and heated in conventional manner to about 1 ,l00 C. The rate of heating up to thistemperature is not critical with respect to product bulk density. The briquettes can, for example, be heated at a rate of less,

than 200 F. perminute to 2,000 F. (1,093" C.) in accordance with Leatham et al. (US. Pat. No. 3,221,082) to reduce spalling and fines production.

The heating rate from l,l00 C. to 1,700 C. is then controlled to less than about 30 C. per minute to improve product density.

Where the dead burning temperature is in excess of 1,700 C., the critical heating rate can be employed up to the actual dead-burning temperature. Preferably the magnesia is heated at a rate of 20 C./minute or less. In general, the lower the heating rate through this critical temperature range, the higher the product density.

EXAMPLES Several l /-inch-diameter briquettes of calcined Mg(Ol-l) were heated in a gas fired furnace to l,400 C. at a rate of about 56 C./minute. Samples of the briquettes were then heated from l,400 to l,700 C. at various rates.

The samples were cooled and then reheated to l,700 C., to

ensure that testing was done with full grain development and maximum density for all samples, in accordance with the following heat schedule: with natural gas-air flame, heat to 500 C. in 1 hour, from 500 to 800 C. in 2 hours, and from 800 to l,000 C. in one-half hour; then with natural gas-oxygen flame, heat from 1,000 to 1,700 C. in 2 hours and hold for 1 hour.

The briquettes were cooled and tested for bulk density by a mercury-displacement method. This method consists of measuring the volume of mercury displaced by a volume of solids. About 2.5 grams of a crushed and screened sample briquette TABLE 1' Heating RatcC./min.

l,400 l H00v C. Densityg./cc.

Substantial improvement in dead-burned magnesia density is achieved .by heating at a rate of less than about 30 C./min. Up until that point lowering the heating rate had little effect on the resultant density.

Although this particular example demonstrates the critical heating rate through a temperature range of l,400 to l,700 C., it has been observed that such heating rate is beneficial with other magnesia samples from a temperature of about 1,l00 C., i.e., heatingat rates greater than 30 C./min. above a temperature ofabout l,l00 C. and beyond produces a relatively low-density product. For example, magnesia briquettes, produced from another calcined Mg(OH) were fired to various temperatures at a rate in excess of 300 C./min. Another portion wasfired to l,700 C. at'about 5-6 C./min. All samples were cooledand refired (slow rate) in accordance with the scheduleof-theprevious example. The bulk density of the slow-firedsample was about 3.40 g./cc. A sample heated to l,000 C. hada density of 3.38 g./cc., about 0.6 percent reduction. Thus heatingrapidly to l,000 C. didnot affect density. substantially since the sample was not subjected to rapid firing above about l,l00 C. However, samples rapidly heated to l,200- C. and l,400 C. had densities of 3.35 g./cc., about.l.5 percent reduction, and 3.30 g./cc., about 3 percent reduction, respectively. These samples were adversely affected by rapid firing from l,000 to 1,200 C. and l,400 C. respectively.

Table llpresents results of actual experiments or results Product bulk density can be substantially improved by controlling the heating rate to less than 30 C./min. through a temperature range of from 1, 1 00 to l,700 C.

What is claimed is:

does not exceed 20 CJminute.

3. The improvement of claim 1 wherein the magnesia is derived from magnesium hydroxide and the magnesia is heated at the specified rate through a temperature range of from about 1,400 to l,700 C. 

2. The improvement of claim 1 wherein the heating rate does not exceed 20* C./minute.
 3. The improvement of claim 1 wherein the magnesia is derived from magnesium hydroxide and the magnesia is heated at the specified rate through a temperature range of from about 1,400* to 1,700* C. 